1. Field of the Invention
The present invention relates to a method for manufacturing a high formable, high strength cold-rolled steel sheet excellent in resistance to secondary working embrittlement.
2. Description of the Related Art
The prior art concerning high-strength cold-rolled steel sheet is extensive. High-strength cold-rolled steel sheet consists of a base steel which is fully decarburized during manufacturing, producing a very low carbon content. To secure formability, C and N dissolved in the base steel are fixed as carbides or nitrides by Ti, Nb, or other fixing elements contained therein. The base steel also comprises dissolved strengthening compositions of Si, P, Mn, etc. to improve strength.
For example, Japanese Laid-Open Patent Publication No. 63-190141 discloses a cold-rolled steel sheet in which Mn and P are added to Ti-containing steel with very low carbon content as described above. In such a cold-rolled steel sheet, adding suitable amounts of Mn and P causes a small amount of dissolved carbon to remain after annealing of the steel sheet, thereby significantly increasing the r-value of the sheet, i.e. Rankford value which is a measure of formability. Additionally, secondary working embrittlement is avoided due to the dissolved carbon remaining at a grain boundary. However, when large amounts of P are added to the above-described steel to produce greater steel strength, resistance to secondary working embrittlement is significantly deteriorated.
The addition of B is well known for improving the resistance of steel to secondary working embrittlement. However, steel sheet to which large amounts of solid-solution strengthening compositions are added tends to become embrittled by those same solid-solution strengthening compositions. Therefore, large amounts of B are required to ensure efficient resistance to secondary working embrittlement. When excessive amounts of B are added, however, formability and hot rolling properties of the steel tend to deteriorate.
In Japanese Patent Publication No. 59-42742, there is proposed a steel to which Si is added as a solid-solution strengthening composition in addition to Mn and P, and B is added to improve resistance to secondary working embrittlement so as to produce a high strength steel with a high r-value. The yield ratio of this cold rolled steel sheet is a very low 60% or less. However, we discovered that when the tensile strength of this high strength cold-rolled steel sheet exceeds 40 kgf/mm.sup.2, containing solid-solution elements such as Si, Mn and P and having a ferrite single phase structure, it is almost impossible to obtain highly formable steel.
The steels described in Japanese Laid-Open Patent Publication No. 63-190141 and Japanese Patent Publication No. 59-42742 can be obtained by subjecting to annealing at a temperature below the Ac.sub.1 transformation temperature to get ferrite single phase structure. Another publications recite methods of increasing steel strength which involve annealing the steel in two phase regions to produce a hard second phase. However, the second phase is merely used for securing the strength of the steel, and there is no consideration regarding formability and resistance to secondary working embrittlement.
A cold-rolled steel sheet possessing a well-balanced array of properties, including high tensile strength of 38 kgf/mm.sup.2 or more, formability and resistance to secondary working embrittlement would be desirable for many applications, including outer panel applications in automobiles and household appliances.